Abstract:
A new monthly volcanic forcing dataset is included in a coupled GCM for a more physically consistent treatment of the stratospheric sulfate aerosol history from explosive volcanism. The volcanic forcing is different from previous versions in that there is an individual evolution of the aerosol for each event. Thus the seasonal and latitudinal dependence of the volcanic aerosol can affect ... global climate in a more realistic way prior to the satellite period, compared to earlier volcanic forcing datasets. Negative radiative forcing from volcanic activity is greatest in the early 20th century prior to 1915 and in the late 20th century after 1960. The combination of volcanic and solar forcing contributes to an early-20th century warming, followed by relative cooling in late 20th century. Consequently, the addition of natural forcing factors to the anthropogenic GHG forcing in late 20th century is required to simulate the observed late 20th century warming.

This data set contains monthly volcanic forcing data for climate modeling from 1890-1999. Aerosol from each event is individually evolved spatially at monthly resolution. Latitude bands are 2.8 degrees. The peak aerosol optical depth for each eruption is scaled from estimates of peak aerosol loading [Stothers 1996; Hofmann and Rosen 1983; Stenchikov et al., 1998] assuming spherical sulfuric acid droplets with a composition of 75% H2SO4 and 25% H2O and a fixed aerosol size distribution (using reff = 0.42 micron [Ammann et al., 2003], a mid-sized aerosol comparable to an average Pinatubo aerosol, Stenchikov et al. [1998]; see also Stothers [2001b] for estimates of reff for individual events). Using the month of the eruption and an estimated peak amount of sulfate aerosol reached through linear buildup after 4 months, aerosol from tropical eruptions is transported in the lower stratosphere from the tropics (25N?25S) into the midlatitudes of the respective winter hemisphere, where it subsequently decays during the summer season. Over the poles, aerosol is rapidly removed during winter, but is advected from midlatitudes in spring and summer after the breakdown of the polar vortex. The e-folding time for decay is set to 12 months in the tropics. The aerosol from high-latitude eruptions is restricted to remain poleward of 30 degrees of the appropriate hemisphere.